The present invention relates to data processing systems such as personal computers and, more particularly, to a data processing system provided with memory card loading means for loading and unloading a memory card therein.
In the data processing system such as a personal computer, RAM, ROM, I/O, etc. are connected with a CPU by common buses and address signals, data, write signals, read signals and so forth via the common buses for their transmission and reception. The structure of such a prior art system is shown schematically in FIG. 19.
Referring to FIG. 19, a CPU 1 is connected with a RAM 2, a ROM 3, an I/O 4, a memory card 5 and like devices via common address and data buses 6 and 7. The I/O 4 is connected with an operation/display system 8 and serves as a man-machine interface. The CPU 1 and the devices noted above are each supplied with predetermined power from a power supply 9.
In this type of system of FIG. 19, it is required that the CPU 1 can access a specific device, such as the RAM 2, ROM 3, I/O 4 or memory card 5, without accessing the other devices. To this end, the address area in the CPU 1 is assigned to the individual devices as shown in FIG. 20, and an address decoder 10 is provided, which generates a chip select signal (CS1 or CS2) designating a specific device to be accessed by decoding the address signal outputted from the CPU 1.
With this structure, when the CPU 1 is accessing, for instance, area A in the address area shown in FIG. 20, the address decoder 10 makes the chip select signal CS2 effective to select the ROM 3. At this time, no other chip select signal is outputted, and the other devices than the ROM 3 do not respond to the write and the read signal from the CPU 1. In the pertinent data processing operation, the CPU 1 reads out and executes successive programs stored in predetermined areas.
Usually, in the system of the above structure the programs have been previously written in the ROM 3. To write the program in the ROM, they are written in a mask for manufacturing the ROM 3. Alternatively, a ROM capable of writing data therein is used to write the programs before its mounting. The ROM may be mounted using a socket, or directly soldered to the circuit board.
In the above prior art system, however, it is very difficult to provide for a different program operation without updating the ROM program. This does not only pose problems when it is desired to increase the versatility of the system but also is very inconvenient for operation confirmation during manufacture. The operation confirmation requires exclusive programs to be stored previously in the ROM in addition to the operation programs that may be executed by the system. When the operation confirmation process is complicated, the operation confirmation programs may be greater in scale than the programs provided for execution by the system. In such a case, a large capacity ROM is necessary. A design with a restricted ROM capacity inevitably results in an extreme limitation of the operation confirmation items obtainable according to programs which can be stored in the ROM.
In a different aspect, there often arises a need for updating a CPU program (i.e., a program stored in the ROM 3) in order to change or expand the system function. Re-writing of the ROM data requires opening a system body housing even when the ROM is capable of data re-writing. To ensure readiness of the operation, mounting of the ROM in a socket is necessary. In a case in which the ROM capable of data re-writing is directly soldered to the circuit board, the replacement of the ROM requires removal of the solder. This operation is cumbersome and requires skill. In such an operation there is a possibility of copper foil separation from the circuit board.
In order to overcome the above drawbacks in the prior art system, there is a proposal to use a non-volatile memory capable of data re-writing, such as a flush EPROM, and allowing programs once stored in the ROM (i.e., flush EPROM) to be updated using an exclusive program for program re-writing after the start of the system.
According to this method, however, the exclusive program for program re-writing can be executed only in the operative state of the system. Further, such exclusive program for program re-writing provides its function only for updating a program that has been stored. This means that a program re-writing operation is dictated before mounting the ROM, such as when storing a new program when the ROM is blank, for instance, during system manufacture. Therefore, it was necessary to mount the ROM in the socket.
In a further aspect, upon failure of a normal program updating operation, it is impossible subsequently to start the system. It is therefore necessary to re-write the ROM data using an exclusive writing means after removing the ROM. This operation of course requires opening the system housing and is therefore, cumbersome, so that the program updating can not be made readily and frequently.